A variety of segmented pipe couplings are well known in the art, the respective coupling segments of such coupling being comprised of castings, typically of ductile iron, whose interior surface is configured to tightly engage the exterior circumference of the pipe as the coupling segments are typically bolted together in close mechanical engagement. One such type of coupling segment is shown in U.S. Pat. No. 4,601,495, assigned to the assignee of the present invention. It includes circumferentially extending keys at their respective opposite sides, which are shaped to be received within complementary grooves cut or otherwise formed in the exterior circumference of the pipe. The keys act to resist axial stresses that are generated in the pipes, with the axial stresses being absorbed by the coupling as a tensile or a compressive stress. A sealing gasket is also provided intermediate the sealing spaced keys.
The present invention is however directed to a segmented pipe coupling which is intended to be secured to a plain ended pipe. That is, a pipe that does not have a circumferential groove for the reception of a complementary key formed along the internal circumference of the coupling. Such plain ended pipe couplings must include some means along their inner circumferential area to tightly engage the exterior surface of the plain ended pipe, and maintain such secure engagement under varying temperature extremes and loads. One such coupling formed of a ductile iron casting intended for utilization in conjunction with high density polyethylene plain ended pipe are the commercially available Styles, 994, 995, and 997 couplings of the Victaulic Company of America, Easton, Pennsylvania. That coupling includes sharp circumferential teeth which engage, and actually cut into the pipe wall as the coupling torque bolts are tightened. A variation of this type of coupling segment includes separate sharp teeth formed in hardened steel cutter members inserted within the inner circumference of the coupling for cutting into the exterior surface of the pipe, as is disclosed in U.S. patent application Ser. No. 08/690,481 filed on Jul. 31, 1996 issued as U.S. Pat. No. 5,911,446 and assigned to the assignee of the present invention. Such sharp circumferential teeth that puncture and bite into the exterior wall of the plain ended pipe are typically required where the coupling is formed of cast ductile iron and the pipe is formed of high density polyethylene. This is necessary for adequate holding strength under a wide temperature range. High density polyethylene has a substantially larger coefficient of thermal expansion than the iron forming the coupling. Thus it will shrink considerably in diameter and length when cooled and expands considerably in diameter and length when heated. Since the ductile iron which forms the coupling has a substantially smaller coefficient of expansion, the high density polyethylene pipe will outshrink the ductile iron coupling when cooled and outexpand the ductile iron coupling when heated. Hence it is necessary for the teeth to actually cut into the exterior wall of the high density polyethylene pipe in order to properly maintain a secure coupling engagement therebetween when the pipe coupling is cooled. This prevents coupling disengagement when the high density polyethylene pipe will shrink more in diameter relative to the ductile iron coupling upon such cooling. Further, the shrinking in pipe length puts a substantial axial load on the couple joint. To hold the joint together, it is required that the coupling stay sufficiently engaged on the reduced diameter of the high density polyethylene pipe, thereby necessitating the utilization of teeth which actually cut into the pipe exterior.
While such prior couplings have provided satisfactory performance in conjunction with plain ended high density polyethylene pipe, it creates certain disadvantages when used in a pipe coupling intended for polyvinylchloride pipe or other pipe materials (e.g., fiber reinforced plastic pipe) which have a lesser coefficient of thermal expansion than high density polyethylene pipe. The sharp circumferential teeth required for coupling to plain ended high density polyethylene pipe would create circumferential notches in the polyvinylchloride pipe. Such notches are detrimental to the pipe's long term performance, since such notches create potential crack initiation points. Similarly when used with fiber reinforced plastic pipe the teeth cut the fibers, which may typically be carbon, glass, or other reinforcing fibers.
U.S. Pat. No. 4,568,112 has recognized the desirability of providing a segmented coupling for polyvinylchloride pipe which avoids the piercing or breaking of the exterior surface of the polyvinylchloride pipe. That patent employs continuous circumferential ribs in alternating combination with a rough machine phonographic type finish. While not cutting into the pipe, such continuous circumferential depressions disadvantageously create sources of high stress concentration within the polyvinylchloride pipe. Further, the circumferential gripping ribs shown in U.S. Pat. No. 4,568,112 contain a 1.degree. taper on the gripping zone of the flange adapter. This taper is in the direction that creates deeper pipe depressions at the back end or outboard end of the flange adapter. Since this is an area of highest stress in the pipe created by the flange adapter, it will be a point of highest stress in applications involving pipe bending and cyclic pressure. Since this is an area where pipe failure oftentimes is initiated, the flange adapter with its 1.degree. taper exacerbates this potential for pipe failure. Another disadvantage of the coupling structure shown in U.S. Pat. No. 4,568,112 is that the two coupling segments do not meet pad to pad. This requires the installer to measure the torque being applied to the bolts as the segments are pulled together and into contact with the pipe surface to insure that they will be at an adequate magnitude to prevent the pipe from being pulled out of the flange, but not at too high a level to overstress the pipe or flange. Relying on the measurement of torque magnitude to achieve a specific bolt load tends to be imprecise due to the inherent imprecision of bolt load generation as well as anticipated variations generated by the installer. Thus it would be desirable to insure proper gripping force without resorting to bolt torque measurement.